The term instantaneous is appropriate because the aggregate is applied over an event. It suggests an interest in the aggregate value over a very small time interval, an instant, much as acceleration is defined in physics over an infinitesimally small time (+R3).
Many temporal query languages perform instantaneous aggregation, others use cumulative aggregation, while still others use a combination of the two. This term will be useful to distinguish between the various alternatives, and is already used by some researchers (+R4,+E3).